A new radiation modality, encapsulated Pd-103 seed sources, is now in clinical use in the treatment of human cancer and radiation. The physical characteristics of this isotope are attractive for its use in permanent implantation. In comparison with Au-198, the low photon energies of Pd-103 (about 21 keV) reduces unwanted radiation exposure. Compared to I-125 (t 1/2 of about 60 days), the 17 day half life of Pd-103 corresponds to a more rapid delivery of radiation dose, which may be more efficacious in controlling tumors with fast growth kinetics. It is likely that the use of Pd-103 will increase as clinical experience is accumulated. It is most probable that Pd-103 has higher radiobiological effectiveness (RBE) than Cs-137. Its RBE may also be greater than that of I-125, with values ranging from 1.0-2.0. No data exists on the RBE of Pd-103. We propose to measure the RBE of Pd-103 relative to Cs-137, for a number of dose rates and for a number of cell lines of different growth kinetics. The RBE of I-125 will be re-examined, in part for a direct comparison with Pd- 103, and in part motivated by the recent discovery of previous dosimetric errors. Permanent implants with short half life radionuclides (e.g. PD-103 or Au- 198) deliver radiation with decreasing dose rate. The radiobiological effect of such time-dose patterns is not well understood. We propose experiments to study the topic, which also have relevance to the radiobiology of radiolabelled immunotherapy. Biophysical models will be examined in comparison with experimental results.